Computer Programming. Basic Control Flow - Loops. Adapted from C++ for Everyone and Big C++ by Cay Horstmann, John Wiley & Sons

Transcription

1 Computer Programming Basic Control Flow - Loops Adapted from C++ for Everyone and Big C++ by Cay Horstmann, John Wiley & Sons

2 Objectives To learn about the three types of loops: while for do To avoid infinite loops and off-by-one errors To understand nested loops and sentinel. 2

3 What Is the Purpose of a Loop? A loop is a statement that is used to: execute one or more statements repeatedly until a goal is reached. Sometimes these one-or-more statements will not be executed at all if that s the way to reach the goal 3

4 The Three Loops in C++ C++ has these three looping statements: while for do 4

5 The while Loop while (condition) statements The condition is some kind of test (the same as it was in the if statement in Chap. 3) 5

6 The while Loop while (condition) statements The statements are repeatedly executed until the condition is false 6

7 The while Loop 1.0 START 2.0 Read input from user, countdown. 3.0 IF the countdown is more than 0 THEN 3.1 Display * 3.2 Decrement countdown by Repeat step 3.0. ELSE 3.4 Go to Step END. 7

8 The while Loop cin >> countdown; while( countdown > 0) cout << "* " ; countdown--; 8

9 The while Loop When doing something repetitive, we want to know when we are done. Example: I want to get at least $20,000 So we set balance >= TARGET But the while loop thinks the opposite: How long I am allowed to keep going? So the correct condition : while (balance < TARGET) 9

10 The while Loop 10

11 Using a Loop to Solve the Investment Problem. The algorithm for an investment problem: 1. Start with a year value of 0 and a balance of $10, Repeat the following steps while the balance is less than $20,000: Add 1 to the year value. Multiply the balance value by 1.05 (a 5 percent increase). 3. Report the final year value as the answer. Repeat.. while in the problem indicates a loop is needed. To reach the goal of being able to report the final year value, adding and multiplying must be repeated some unknown number of times. 11

12 Using a Loop to Solve the Investment Problem. The statements to be controlled are: Incrementing the year variable Updating the balance variable using a const for the RATE year++; balance = balance * (1 + RATE / 100); 12

13 Using a Loop to Solve the Investment Problem. The condition, which indicates when to stop executing the statements, is this test: (balance < TARGET) 13

14 Using a Loop to Solve the Investment Problem. Here is the complete while statement: while (balance < TARGET) year++; balance = balance * (1 + RATE / 100); 14

15 Flowchart of the Investment Calculation s while Loop 15

16 The Complete Investment Program #include <iostream> using namespace std; int main() const double RATE = 5; const double INITIAL_BALANCE = 10000; const double TARGET = 2 * INITIAL_BALANCE; double balance = INITIAL_BALANCE; int year = 0; while (balance < TARGET) year++; balance = balance * (1 + RATE / 100); cout << "The investment doubled after " << year << " years." << endl; return 0; 16

17 Program Run 1. Check the loop condition balance = year = 0 while (balance < TARGET) year++; balance = balance * (1 + rate / 100 ); 17

18 Program Run 1. Check the loop condition balance = year = 0 while (balance < TARGET) year++; The condition is true balance = balance * (1 + rate / 100 ); 18

19 Program Run 1. Check the loop condition balance = year = 0 while (balance < TARGET) year++; The condition is true balance = balance * (1 + rate / 100 ); 2. Execute the statements in the loop balance = year = 1 while (balance < TARGET) year++; balance = balance * (1 + rate / 100 ); 19

20 Program Run 3. Check the loop condition again balance = year = 1 while (balance < TARGET) year++; The condition is still true balance = balance * (1 + rate / 100 ); 20

21 Program Run 3. Check the loop condition again balance = year = 1 while (balance < TARGET) year++; The condition is still true balance = balance * (1 + rate / 100 ); 4. Execute the statements in the loop balance = year = 2 while (balance < TARGET) year++; balance = balance * (1 + rate / 100 ); 21

22 Program Run 3. Check the loop condition again balance = year = 2 while (balance < TARGET) year++; The condition is still true balance = balance * (1 + rate / 100 ); 4. Execute the statements in the loop balance = year = 3 while (balance < TARGET) year++; balance = balance * (1 + rate / 100 ); 22

23 Program Run This process continues for 15 iterations 23

24 Program Run The final output indicates that the investment doubled in 15 years. 24

25 Program Run 25

26 Program Run 26

27 More while Examples Skip the examples? NO YES 27

28 More while Examples For each of the following, do a hand-trace 28

29 Example of Normal Execution while loop to hand-trace What is the output? i = 5; while (i > 0) cout << i << " "; i--; 29

30 When i Is 0, the Loop Condition Is false, and the Loop Ends while loop The output i = 5; while (i > 0) cout << i << " "; i--; correct? OR

31 Example of a Problem An Infinite Loop while loop to hand-trace What is the output? i = 5; while (i > 0) cout << i << " "; i++; 31

32 Example of a Problem An Infinite Loop i is set to 5 The i++; statement makes i get bigger and bigger the condition will never become false an infinite loop while loop The output never ends i = 5; while (i > 0) cout << i << " "; i++;

33 Another Normal Execution No Errors while loop to hand-trace i = 5; while (i > 5) cout << i << " "; i--; What is the output? 33

34 Another Normal Execution No Errors while loop i = 5; while (i > 5) cout << i << " "; i--; There is (correctly) no output The expression i > 5 is initially false, so the statements are never executed. 34

35 Another Normal Execution No Errors while loop i = 5; while (i > 5) cout << i << " "; i--; There is (correctly) no output This is not a error. Sometimes we do not want to execute the statements unless the test is true. 35

36 Normal Execution with Another Programmer s Error while loop to hand-trace What is the output? i = 5; while (i < 0) cout << i << " "; i--; 36

37 Normal Execution with Another Programmer s Error The programmer probably thought: Stop when i is less than 0. However, the loop condition controls when the loop is executed - not when it ends. while loop i = 5; while (i < 0) cout << i << " "; i--; Again, there is no output 37

38 A Very Difficult Error to Find (especially after looking for it for hours and hours!) while loop to hand-trace What is the output? i = 5; while (i > 0); cout << i << " "; i--; 38

39 A Very Difficult Error to Find (especially after looking for it for hours and hours!) Another infinite loop caused by a single character: That semicolon causes the while loop to have an empty body which is executed forever. The i in (i > 0) is never changed. while loop There is no output! i = 5; while (i > 0); cout << i << " "; i--; ; 39

40 Common Error Infinite Loops Forgetting to update the variable used in the condition is common. In the investment program, it might look like this. year = 1; while (year <= 20) balance = balance * (1 + RATE / 100); The variable year is not updated in the body 40

41 Common Error Infinite Loops Another way to cause an infinite loop: Typing on autopilot Typing ++ when you meant to type -- is a real problem, especially when it s 3:30 am! year = 20; while (year > 0) balance = balance * (1 + RATE / 100); year++; 41

42 A Not Really Infinite Infinite Loop Due to what is called wrap around, the previous loop will end. At some point the value stored in the int variable gets to the largest representable positive integer. When it is incremented, the value stored wraps around to be a negative number. That definitely stops the loop! 42

43 Common Error Are We There Yet? When doing something repetitive, most of us want to know when we are done. For example, you may think, I want to get at least $20,000, and set the loop condition to while (balance >= TARGET) wrong test 43

44 Common Error Are We There Yet? But the while loop thinks the opposite: How long am I allowed to keep going? What is the correct loop condition? while ( ) 44

45 Common Error Are We There Yet? But the while loop thinks the opposite: How long am I allowed to keep going? What is the correct loop condition? while (balance < TARGET) In other words: Keep at it while the balance is less than the target. 45

46 Common Error Off-by-One Errors In the code to find when we have doubled our investment: Do we start the variable for the years at 0 or 1 years? Do we test for < TARGET or for <= TARGET? 46

47 Common Error Off-by-One Errors Maybe if you start trying some numbers and add +1 or -1 until you get the right answer you can figure these things out. It will most likely take a very long time to try ALL the possibilities. No, just try a couple of test cases (while thinking). 47

48 Use Thinking to Decide! Consider starting with $100 and a RATE of 50%. We want $200 (or more). At the end of the first year, the balance is $150 not done yet At the end of the second year, the balance is $225 definitely over TARGET and we are done. We made two increments. What must the original value be so that we end up with 2? Zero, of course. 48

49 Use Thinking to Decide! Another way to think about the initial value is: Before we even enter the loop, what is the correct value? Most often it s zero. 49

50 < vs. <= (More Thinking) Figure out what you want: we want to keep going until we have doubled the balance So you might have used: (balance < TARGET) 50

51 < vs. <= (More Thinking) But consider, did you really mean: to have at least doubled Exactly twice as much would happen with a RATE of 100% - the loop should top then So the test must be (balance <= TARGET) 51

52 The for Loop 52

53 The for Loop for (initialization; check; update) statements The check is some kind of test (the same as the condition in the while loop) It is usually used to cause the statements to happen a certain number of times. 53

54 The for Loop for (initialization; check; update) statements The statements are repeatedly executed until the check is false. 54

55 The for Loop for (initialization; check; update) statements The initialization is code that happens once, before the check is made, in order to set up for counting how many times the statements will happen. 55

56 The for Loop for (initialization; check; update) statements The update is code that causes the check to eventually become false. Usually it s incrementing or decrementing the loop variable. 56

57 The for Loop Is Better than while for Doing Certain Things Consider this code which write the values 1 through 10 on the screen: int count = 1; // Initialize the counter while (count <= 10) // Check the counter cout << count << endl; count++; // Update the counter initialization check statements update 57

58 The for Loop Is Better than while for Doing Certain Things Doing something a certain number of times or causing a variable to take on a sequence of values is so common, C++ has a statement just for that: for (int count = 1; count <= 10; count++) cout << count << endl; initialization check statements update 58

59 Execution of a for Statement Consider this for statement: int count; for (counter = 1; count <= 10; counter++) cout << counter << endl; 59

60 60

61 Scope of the Loop Variable Part of the for or Not? The loop variable when defined as part of the for statement cannot be used before or after the for statement it only exists as part of the for statement and should not need to be used anywhere else in a program. 61

62 Solving a Problem with a for Statement Earlier we determined the number of years it would take to (at least) double our balance. Now let s see the interest in action: We want to print the balance of our savings account over a five-year period. The over a five-year period indicates that a for loop should be used. Because we know how many times the statements must be executed we choose a for loop. 62

63 Solving a Problem with a for Statement The output should look something like this: 63

64 The for Loop Flowchart of the investment calculation s while loop Easily written using a for loop 64

65 Solving a Problem with a for Statement Two statements should happen five times. So use a for statement. They are: update balance print year and balance for (int year = 1; year <= nyears; year++) // update balance // print year and balance 65

66 The Modified Investment Program Using a for Loop #include <iostream> #include <iomanip> using namespace std; int main() const double RATE = 5; const double INITIAL_BALANCE = 10000; double balance = INITIAL_BALANCE; int nyears; cout << "Enter number of years: "; cin >> nyears; cout << fixed << setprecision(2); for (int year = 1; year <= nyears; year++) balance = balance * (1 + RATE / 100); cout << setw(4) << year << setw(10) << balance << endl; return 0; 66

67 The Modified Investment Program Using a for Loop A run of the program: Enter number of years:

68 More for Examples For each of the following, do a hand-trace. 68

69 Example of Normal Execution for loop to hand-trace for (int i = 0;i <= 5;i++) cout << i << " "; What is the output? 69

70 Example of Normal Execution for loop for (int i = 0;i <= 5;i++) cout << i << " "; The output Note that the output statement is executed six times, not five 70

71 Example of Normal Execution Going in the Other Direction for loop to hand-trace What is the output? for (int i = 5;i >= 0;i--) cout << i << " "; 71

72 Example of Normal Execution Going in the Other Direction Again six executions of the output statement occur. for loop for (int i = 5;i >= 0;i--) cout << i << " "; The output

73 Example of Normal Execution Taking Bigger Steps for loop to hand-trace for (int i = 0; i < 9; i += 2) cout << i << " "; What is the output?

74 Example of Normal Execution Taking Bigger Steps for loop for (int i = 0; i < 9; i += 2) cout << i << " "; The output The step value can be added to or subtracted from the loop variable. Here the value 2 is added. There are only 5 iterations, though. 74

75 Infinite Loops Can Occur in for Statements The danger of using == for loop to hand-trace for (int i = 0; i!= 9; i += 2) cout << i << " "; and/or!= What is the output? 75

76 Infinite Loops Can Occur in for Statements = = and!= are best avoided in the check of a for statement for loop for (int i = 0; i!= 9; i += 2) cout << i << " "; The output never ends

77 Example of Normal Execution Taking Even Bigger Steps for loop to hand-trace for (int i = 1; i <= 20; i *= 2) cout << i << " "; What is the output? The update can be any expression 77

78 Example of Normal Execution Taking Even Bigger Steps for loop for (int i = 1; i <= 20; i *= 2) cout << i << " "; The output The step can be multiplicative or any valid expression 78

79 The do Loop The while loop s condition test is the first thing that occurs in its execution. The do loop (or do-while loop) has its condition tested only after at least one execution of the statements. 79

80 The do Loop This means that the do loop should be used only when the statements must be executed before there is any knowledge of the condition. This also means that the do loop is the least used loop. 80

81 The do Loop What problems require something to have happened before the testing in a loop? Getting valid user input is often cited. Here is the flowchart for the problem in which the user is supposed to enter a value less than 100 and processing must not continue until they do. 81

82 The do Loop Here is the code: int value; do cout << "Enter a value < 100"; cin >> value; while (value >= 100); In this form, the user sees the same prompt each time until the enter valid input. 82

83 The do Loop In order to have a different, error prompt that the user sees only on invalid input, the initial prompt and input would be before a while loop: int value; cout << "Enter a value < 100"; cin >> value; while (value >= 100); cout << "Sorry, that is larger than 100\n" << "Try again: "; cin >> value; Notice what happens when the user gives valid input on the first attempt: nothing good. 83

84 Nested Loops For each hour, 60 minutes are processed a nested loop. 84

85 Nested Loops Nested loops are used mostly for data in tables as rows and columns. The processing across the columns is a loop, as you have seen before, nested inside a loop for going down the rows. Each row is processed similarly so design begins at that level. After writing a loop to process a generalized row, that loop, called the inner loop, is placed inside an outer loop. 85

86 Nested Loops Write a program to produce a table of powers. The output should be something like this: 86

87 Nested Loops The first step is to solve the nested loop. There are four columns and in each column we display the power. Using x to be the number of the row we are processing, we have (in pseudo-code): You would test that this works in your code before continuing. If you can t correctly print one row, why try printing lots of them? 87

88 Nested Loops Now, putting the inner loop into the whole process we have: (don t forget to indent, nestedly) 88

89 The Complete Program for Table of Powers #include <iostream> #include <iomanip> #include <cmath> using namespace std; int main() const int NMAX = 4; const double XMAX = 10; // Print table header for (int n = 1; n <= NMAX; n++) cout << setw(10) << n; cout << endl; for (int n = 1; n <= NMAX; n++) cout << setw(10) << "x "; // print x cout << endl << endl; 89

90 The Complete Program for Table of Powers // Print table body for (double x = 1; x <= XMAX; x++) // Print table row for (int n = 1; n <= NMAX; n++) cout << setw(10) << pow(x, n); cout << endl; return 0; The program run would be: 90

91 More Nested Loop Examples The loop variables can have a value relationship. In this example the inner loop depends on the value of the outer loop. for (i = 1; i <= 4; i++) for (j = 1; j <= i; j++) cout << "*"; cout << endl; The output will be: * ** *** **** 91

92 More Nested Loop Examples for (i = 1; i <= 4; i++) for (j = 1; j <= i; j++) cout << "*"; cout << endl; j is the number of columns (or the line s length), which depends on the line number, i j stops at: i i i i line num. i 1 * i 2 * * i 3 * * * i 4 * * * * i represents the row number or the line number 92

93 More Nested Loop Examples In this example, the loop variables are still related, but the processing is a bit more complicated. for (i = 1; i <= 3; i++) for (j = 1; j <= 5; j++) if (i + j % 2 == 0) cout << "*"; else cout << " "; cout << endl; The output will be: * * * * * * * * 93

94 Processing Input When and/or How to Stop? or be stopped! 94

95 Processing Input When and/or How to Stop? We need to know, when getting input from a user, when they are done. One method is to hire a sentinel (as shown) or more correctly choose a value whose meaning is STOP! As long as there is a known range of valid data points, we can use a value not in it. 95

96 Processing Input When and/or How to Stop? We will write code to calculate the average of some salary values input by the user. How many will there be? That is the problem. We can t know. But we can use a sentinel value, as long as we tell the user to use it, to tell us when they are done. Since salaries are never negative, we can safely choose -1 as our sentinel value. 96

97 Processing Input When and/or How to Stop? In order to have a value to test, we will need to get the first input before the loop. The loop statements will process each non-sentinel value, and then get the next input. Suppose the user entered the sentinel value as the first input. Because averages involve division by the count of the inputs, we need to protect against dividing by zero. Using an if-else statement from Lecture 3 will do. 97

98 The Complete Salary Average Program #include <iostream> using namespace std; int main() double sum = 0; int count = 0; double salary = 0; // get all the inputs cout << "Enter salaries, -1 to finish: "; cin >> salary; while (salary!= -1) // process input sum = sum + salary; count++; // get next input cin >> salary; 98

99 The Complete Salary Average Program // process and display the average if (count > 0) double average = sum / count; cout << "Average salary: " << average << endl; else cout << "No data" << endl; return 0; A program run: Enter salaries, -1 to finish: Average salary: 20 99

100 Using Failed Input for Processing Sometimes is it easier and a bit more intuitive to ask the user to Hit Q to Quit instead or requiring the input of a sentinel value. Sometimes picking a sentinel value is simply impossible if any valid number is allowed, which number could be chosen? 100

101 Using Failed Input for Processing In the previous chapter we used cin.fail() to test if the most recent input failed. Note that if you intend to take more input from the keyboard after using failed input to end a loop, you must reset the keyboard with cin.clear(). 101

102 Using Failed Input for Processing If we introduce a bool variable to be used to test for a failed input, we can use cin.fail() to test for the input of a Q when we were expecting a number: 102

103 Using Failed Input for Processing cout << "Enter values, Q to quit: "; bool more = true; while (more) cin >> value; if (cin.fail()) more = false; else // process value here cin.clear() // reset if more input is to be taken 103

104 Using Failed Input for Processing Using a bool variable in this way is disliked by many programmers. Why? cin.fail is set when >> fails. This allows the use of an input itself to be used as the test for failure. Again note that if you intend to take more input from the keyboard, you must reset the keyboard with cin.clear. 104

105 Using Failed Input for Processing Using the input attempt directly we have: cout << "Enter values, Q to quit: "; while (cin >> value) // process value here cin.clear(); 105

106 Chapter Summary Loops execute a block of code repeatedly while a condition remains true. The for loop is used when the loop body must be executed at least once. Nested loops are commonly used for processing tabular structures. A sentinel value denotes the end of data set, but it is not part of the data. We can use a Boolean variable to control a loop. Set the variable to true before entering the loop, then set it to false to leave the loop. 106